1. Field of the Invention
The present invention relates to an installation for producing oil from an off-shore deposit, of the type comprising a semi-submersible platform, at least one riser connecting the platform to the sea bed F, and means of tensioning the riser.
2. Description of the Related Art
Semi-submersible platforms are intended for oil production in very deep seas or oceans. They comprise a hull supported by legs, the bottoms of which are connected to a hollow base. The legs have buoyancy boxes. The base and the buoyancy boxes provide the platform with buoyancy and stability. The hull, fixed on the legs, is kept above the surface of the sea while the installation is in production.
One or more of what are commonly known as risers connect the platform to the sea bed. These risers consist of metal tubes.
Their length, which essentially corresponds to the depth of the production site is commonly 1200 m, and their weight is of the order of 100 tons.
To prevent the risers from breaking under the action of transverse currents, it is known practice to provide means of tensioning them. These tensioning means exert a force which corresponds to approximately one to two times the weight of the riser.
Because the platform remains afloat, it is subjected, on the one hand, to the variations in water level due to the tide, and, on the other hand, to movements associated with the heave. In consequence, the means of tensioning the risers must make it possible to compensate for the vertical oscillation of the platform over time. The maximum vertical oscillation is commonly from 4 to 12 m.
In current installations, the means of tensioning the risers comprise hydropneumatically operated rams arranged between the top end of the riser and the platform. These rams need to have a long enough stroke that they can compensate for the relative displacement between the top end of the riser and the platform. Furthermore, these rams have to be powerful enough that they can withstand the hauling force involved in tensioning the riser.
Thus, it will be understood that the rams currently in use are very bulky and employ complex technology.
The object of the invention is to provide a production installation in which the tensioning of each riser does not require the use of complex and bulky means on the hull of the platform.
To this end, the subject of the invention is an installation for producing oil from an off-shore deposit, of the aforementioned type, characterized in that the tensioning means comprise, for each riser, at least one submerged float connected to a point on the main run of the riser for hauling it towards the surface, and a mechanism for hauling the riser, which mechanism is installed on the platform and applied to the top end of the riser.
According to particular embodiments, the invention comprises one or more of the following features:
each float is dimensioned to apply to the riser a hauling force which exceeds the hauling force applied by the top-end hauling mechanism;
the float is dimensioned to apply to the riser a hauling force which is between 1 and 3 times the weight of the riser;
the platform comprises a submerged base and a hull which is out of the water and connected by legs, each float being arranged at the depth of the base, which base comprises means for the vertical guidance of each float;
the base comprises, for each float, a vertical passage through which the float can move axially;
means for bringing the float into abutment against the platform in the upwards direction;
each float has a through conduit through which the associated riser runs;
the means providing the link between each float and the associated riser comprises a ball joint;
the ball joint comprises a concave annular seat secured to the float in the axial conduit and a flange with a convex surface borne by the riser, the flange being pressed against the concave seat in order to apply tension to the riser;
the through conduit has a diameter greater than three times the diameter of the riser; and
the top-end hauling mechanism comprises at least one hydropneumatic ram which, at each end, has a series of block-and-tackle pulleys over which at least one hauling line applied to the riser is engaged.
Other subjects of the invention are processes for installing a riser of an installation of the aforementioned type, characterized in that it comprises:
a bringing the float vertically into abutment against the platform;
b immersing the riser with its lower end held some distance from the sea bed;
c weighing the platform down with ballast;
d lowering the riser and connecting it to the sea bed;
e releasing the float from abutment with the platform; and
f removing the ballast from the platform.
According to one particular embodiment, the process comprises:
a bringing the float into abutment against the platform;
b immersing the riser with its lower end held some distance from the sea bed;
c sinking the float by placing ballast on the float;
d lowering the riser and connecting it to the sea bed;
e releasing the float from abutment with the platform; and
f removing the ballast weighing down on the float.